Java Stack & Queue

In this page:

Working with Stack (LIFO)
Stack Search and Size
Working with Queue (FIFO)
Modern Stack and Queue with Deque
Handling Queue Edge Cases

Working with Stack (LIFO)

Stack (a legacy Vector subclass) provides LIFO (last-in, first-out) behavior through push() and pop() — the most recently added element is always the first one removed, like a stack of plates.

Example: Working with Stack (LIFO)

java

import java.util.Stack;
public class Main {
	public static void main(String[] args) {
		Stack<Integer> stack = new Stack<>();
		stack.push(1);
		stack.push(2);
		System.out.println(stack.pop()); // 2: last in, first out
	}
}

Stack Search and Size

Stack also supports search() to find an element's 1-based distance from the top, and size()/isEmpty() for capacity checks — useful when you need to inspect the stack without popping through it.

Example: Stack Search and Size

java

import java.util.Stack;
public class Main {
	public static void main(String[] args) {
		Stack<Integer> stack = new Stack<>();
		stack.push(10);
		stack.push(20);
		System.out.println(stack.search(10)); // 1-based distance from top
		System.out.println(stack.size());
	}
}

Working with Queue (FIFO)

Queue provides FIFO (first-in, first-out) behavior through offer() and poll() — the first element added is always the first one removed, like people waiting in line.

Example: Working with Queue (FIFO)

java

import java.util.Queue;
import java.util.LinkedList;
public class Main {
	public static void main(String[] args) {
		Queue<Integer> queue = new LinkedList<>();
		queue.offer(1);
		queue.offer(2);
		System.out.println(queue.poll()); // 1: first in, first out
	}
}

Modern Stack and Queue with Deque

ArrayDeque is generally preferred over the older Stack and LinkedList-as-queue for both roles today, since it's faster and was explicitly designed as a modern double-ended queue implementation.

Example: Modern Stack and Queue with Deque

java

import java.util.ArrayDeque;
import java.util.Deque;
public class Main {
	public static void main(String[] args) {
		Deque<Integer> deque = new ArrayDeque<>(); // preferred over Stack/LinkedList
		deque.push(1);
		deque.push(2);
		System.out.println(deque.pop());
	}
}

Handling Queue Edge Cases

Calling pop()/poll() on an empty stack or queue either throws an exception or returns null depending on which method you use — check isEmpty() first, or use the peek/poll variants that return null instead of throwing.

Example: Handling Queue Edge Cases

java

import java.util.Queue;
import java.util.LinkedList;
public class Main {
	public static void main(String[] args) {
		Queue<Integer> queue = new LinkedList<>();
		System.out.println(queue.poll()); // null: empty queue, no exception
		System.out.println(queue.isEmpty());
	}
}

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Java Stack & Queue

Working with Stack (LIFO)

Stack Search and Size

Working with Queue (FIFO)

Modern Stack and Queue with Deque

Handling Queue Edge Cases

Chapter Quiz — Complete all 17 topics to unlock

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